This application claims Paris Convention priority of DE 102 03 279.3 filed Jan. 29, 2002 the complete disclosure of which is hereby incorporated by reference.
The invention concerns a method for influencing the homogeneous static magnetic field B0 in the direction of a z axis in a nuclear magnetic resonance (NMR) apparatus having disturbances caused by superconducting components of radio frequency (RF) coils for receiving NMR signals of a sample in a measuring volume of the NMR apparatus, wherein the superconducting components of the RF coils project past the RF active region of the sample in the z direction, and wherein the disturbances effect the z component of the B0 field in the RF active region of the sample. The invention also concerns an NMR (nuclear magnetic resonance) resonator with at least one RF (radio frequency) coil for emitting and/or receiving RF signals at one or more desired resonance frequencies into and/or from a measuring sample in an investigational volume of an NMR apparatus, disposed about a coordinate origin (x,y,z=0), having means for generating a homogeneous magnetic field B0 in the direction of a z axis, wherein superconducting conductor structures of the NMR resonator are disposed at a radial (x,y) separation from the measuring sample, and wherein the superconducting conductor structures project past the RF active part of the measuring sample in the z direction.
A corresponding device is disclosed in the document DE 101 50 131.5-33 (cited ref. [6]), which does not constitute prior art with respect to the present invention.
NMR is a very distinctive analysis method which is, however, not very sensitive. According to prior art, it is possible to considerably improve the S/N (signal to noise) ratio through use of cooled and in particular superconducting radio frequency coils (see reference [1]). The main problem involved in the use of superconductors in NMR receiving systems (RF receiving coil) is their static magnetization. It can, if not controlled, cause field disturbances within the sample of such a strength that the line width becomes unsuitably large. A number of methods to minimize this undesired magnetization have been published: [2], [3], [4]. The described methods are, however, complicated and have further disadvantages which are described below. In particular, with coils according to [1], substantial disturbing fields are produced in response to subsequent transverse magnetic fields.
There are two classes of coil arrangements (described in [5] and [6]) which, in addition to other advantages, are immune to such disturbances and are therefore superior to the above-mentioned coil types [1], also when methods [2,3] are used.
Despite their advantages, if the coils of [5] or [6] are not uniformly magnetized, they lose their favorable properties.
As shown in detail in [2], global magnetization of a type II superconductor results from induced currents which flow in closed paths within the superconductor. These are determined by the history of the superconductor and, as long as the external conditions do not change, remain constant for a practically unlimited time due to the zero resistance of the superconductor. These currents generate a magnetic field outside of the superconductor which can produce strong undesired field disturbances in the sample volume.
Disturbances in the spectra due to inhomogeneities of the static magnetic field caused by superconducting RF coils have been conventionally minimized using the following strategies:
A. Minimization of Disturbances in the Spectra According to Prior Art through
A1 minimization of the maximum possible size of magnetization (through subdivision of the coil into sufficiently narrow strips [1], [5]).
A2 preventing the generation of any possible residual magnetization.
In all conventional methods, the superconductor is always cooled in the magnetic field so that the undisturbed form of the magnetic field lines is always frozen therein during the superconducting transition of the type II superconductor. This minimizes the production of field lines which differ from the original homogeneous B0 field dependence, and therefore also minimizes disturbances of the homogeneity of the magnetic field. The patent document [4] even recommends carrying out this cooling process as slowly as possible so that the B0 field is frozen as uniformly as possible and without disturbances.
A3 Post-treatment of the superconducting coil with a sequence of decreasing transverse magnetic fields for xe2x80x9cdemagnetizationxe2x80x9d [2], [3] (a current structure with closely adjacent opposite current regions is thereby induced such that the sum of the individual magnetic field contributions cancels to a good approximation).
All previous methods are based on a common effort of minimizing or completely eliminating the effective magnetic susceptibility and therefore the magnetization of the superconducting coil(s) and the external magnetic fields produced thereby to minimize the magnetic fields which they generate in the sample.
The final aim of all methods and devices discussed herein is actually to eliminate the NMR disturbances in the spectra produced by the superconducting (SC) coils. We will see that this is not necessarily equivalent to minimization or elimination of the effective susceptibility or minimization/elimination of the additional fields produced by the SC coils. This subtle but very substantial difference in the goals has been completely ignored in prior art [2], [3], [4] and all previous approaches therefore concentrated on the elimination or reduction of all additional fields produced by the SC coils. If this difference is analyzed precisely, completely different solutions of this central object become possible, namely elimination or significant reduction of NMR disturbances in the spectra.
It is therefore the object of the present invention to transfer radio frequency coils with homogeneous transverse saturation magnetization, i.e. coils in accordance with [5] and a subset of the coils according to [6], from any magnetization state into a state in which the NMR relevant field disturbances are essentially eliminated.
This object is achieved in accordance with the inventive method in a surprisingly simple and effective fashion in that the superconducting components of the RF coils are exposed to an additional magnetic field which is sufficiently strong that, during application of this additional magnetic field, all superconducting structures in the superconducting components of the RF coils disposed in the vicinity of the RF active region of the sample are magnetized to a maximum possible extent, wherein their magnetization along the z axis and transverse with respect to B0 assumes a value which is substantially constant and different from zero.
This object is also achieved in accordance with the invention in a device having superconducting conductor structures which are magnetized transverse to the B0 direction after application of an additional magnetic field with maximum magnetization of all superconducting conductor structures, the magnetization along the z axis having a substantially constant value different than zero.
Many of the coils having superconductors which extend sufficiently beyond the RF region ([5], [6]) can be freed from magnetic disturbances using the present invention in a simple and rapid fashion.
In contrast to the conventional methods [2], [3], [4], the present invention eliminates NMR disturbances through homogenisation, in the simplest form through maximizing the magnetization of the superconductor and associated maximizing the magnetic disturbing fields in the sample. This fact would appear at first glance to be completely contradictory and is incompatible with the known methods. However, as described below, it produces the desired success in the coil class under discussion. Moreover, the inventive method is very simple compared to conventional methods: it requires simple or no additional hardware and leads to very robust and reproducible results.
The essentials of the present invention are explained below:
B Inventive Method
B1 The coils of [6], having superconductors whose critical currents are isotropic, in the embodiments 3 or 4 of [6] or birdcage coils according to arrangements such as ([5], FIG. 1), are hereby designated xe2x80x9cCHTSMxe2x80x9d (Coils with Homogeneous Transverse Saturation Magnetisation). Such coil arrangements 8 are shown in FIG. 2, together with the sample tube 7.
As shown in [6], it is not essential for shimming that all field components of an additional disturbing field dB in the sample generated by the SC coils become small. ONLY the Bz component must be minimized or at least homogenized. The coils described in [6] are based on this finding. They generate a transverse magnetization density in response to external transverse fields which is homogeneous along B0. This leads, irrespective to the size of transverse magnetization, to a negligible Bz component at the sample location. It is thereby important that the transverse magnetization density is constant over z.
B2 In accordance with the invention, the required z homogeneity of transverse magnetization can be obtained for CHTSM coils without utilizing the methods of A2 or A3. It is very important to realize that a constant magnetization over z is important and not the absolute size of that magnetization. If a constant dependence is obtained, the goal is achieved. For CHTSM coils, this is obtained in accordance with the invention in that the constant nature (in z) of the maximum (saturation) magnetization is used. This is inherent in this class of coils.
B3 It is possible to generate a homogeneous transverse magnetization in z following any magnetization state (having, in general, a non-uniform magnetization in z: see FIG. 1a), in that all partial structures are magnetized not to a minimum degree but rather homogeneously, in particular, to a maximum. This also maximizes the disturbing fields dB at the sample location. These are, in general, not homogeneous (large inhomogeneities in the x,y plane) and can be very large, as shown in FIG. 1b. 
B4 This maximum magnetization of SC is provided by the geometry (width, thickness) of the individual elements and by the critical current density of the superconductor along the longest dimension of the elements.
B5 For coils of corresponding design (CHTSM coils, B1 above), this produces a large (maximum intensity) but very precisely defined transverse magnetization MT which is uniform along z (FIG. 3).
B6 This eliminates the NMR relevant disturbances in accordance with [6] (see FIG. 1b).
B7 This lack of disturbances remains for all later times in accordance with [6] unless excessively strong inhomogeneous transverse magnetic fields are applied.
As stated above, one single very strong magnetic pulse or application of a strong transverse magnetic field is sufficient for the CHTMS coil types to eliminate the relevant NMR disturbances. This has the following advantages over the conventional methods [2,3]:
C. Advantages
C1 One single pulse is sufficient. Complicated pulse shapes and sequences are not required.
C2 The strength of the pulse must be sufficiently large. Exact adjustments to the SC material used are not required.
C3 This saturation can be carried out very fast since decreasing sequences such as in [2,3] are not required.
C4 No precise devices are required.
C5 Due to the above, the magnetic treatment can also be carried out before/during/between pulse sequences.
C6 In the simplest case, no additional devices are required for the method.
In a simple embodiment, the method can be effected using a transverse field coil 21. FIG. 5 represents the magnetization field BT. It is important that the change dBT be sufficiently large. This can be effected in different ways: in the simplest case, through switching off the previously switched-on current (FIGS. 5b, 5c). Irrespective of the initial magnetization (fully drawn or broken lines in FIG. 5d) the transverse magnetization MT approaches a maximum, well-defined value MT MAX which is constant along z. The magnetic fields BM produced in the sample 7 therefore also approach a maximum value which is constant along z.
In view of C1, C2, C4, implementation is very easy through mechanical tilting of the superconducting coil arrangement. One single motion or one single reciprocating motion having an abutment or stop is sufficient. Control of the amplitude is not necessary (it must be ensured that a minimum amplitude is obtained) as is shown, in the simplest form, in FIG. 6. The result is independent of the initial magnetization state (differently broken lines in FIG. 6d, all resulting in MT MAX).
The same argumentation as in B5 above permits very simple implementation of the method in that no special device is required: The probe head is withdrawn from the magnet (to a sufficient extent) in the cold state (superconducting (SC) coils below the critical temperature Tc), and then re-inserted. As a result thereof, the radial B0 components (which are present outside of the homogeneous region (xe2x80x9cplateauxe2x80x9d) of the magnet due to the outward curvature of the field lines (see FIG. 18) automatically subject the SC coil to a very strong transverse field change dBT eff (FIG. 19) which increases monotonically during insertion. (Or the probe head is cooled down below Tc only outside of the magnet in contrast to [4] and then inserted in the cold state).
FIG. 19 shows the dependence of MT(z) on the motion of the SC coil during insertion into the magnet. This method is advantageous due to the rotational symmetry about z, since not only planar structures are saturated in one orientation (e.g. parallel to x) but also those with perpendicular orientation (parallel to y) or any other orientation.
As illustrated above, the method constitutes a completely new, very effective robust and inexpensive way of conditioning superconducting NMR coils such that they cause no disturbances in the NMR spectrum. These methods and devices form, together with the CHTSM coils, an important component for broad application of SC technology in NMR receiving coils.
A preferred variant of the inventive method is characterized in that the temporal dependence of the additional magnetic field is selected such that the superconducting structures remain maximally magnetized after application of the additional magnetic field, thereby maximizing the magnetic field in the sample region caused thereby. This final state of the RF coil magnetization is particularly easy to produce, in particular, with one single magnetic pulse.
In a particularly preferred method variant, the additional magnetic field is applied transverse to the static magnetic field B0. This is the simplest geometry to obtain the inventive transverse magnetization of the superconducting structures, wherein the entire additional magnetic field contributes to the transverse magnetization.
The above method variant can be realized in a particularly simple fashion in that the effective transverse additional magnetic field acting on the superconductor is produced through tilting of the superconducting coil or coil arrangement about an axis which is not parallel to the B0 direction. This utilizes the static magnetic field B0 for magnetization such that additional field-generating structures are not required.
In a method variant, the additional magnetic field can be generated by a field coil. This permits exactly defined introduction of the additional magnetic field e.g. as a sequence of magnetic pulses.
In a further preferred method variant, the additional magnetic field is generated by two or more field coils which are activated one after the other such that all components of the superconducting receiving coil arrangement which are disposed in the vicinity of the RF active region of the sample are maximally magnetized at least once during processing. This guarantees homogeneous magnetization of the superconducting structures even when the superconducting structures are oriented with a spatial distribution.
Another preferred method variant is characterized in that the effective transverse additional magnetic field which acts on the superconductor is generated by successive tilting of the superconducting receiving coil or receiving coil arrangement about two axes which are not parallel to the B0 direction. This also guarantees homogeneous magnetization of the superconducting structures when the superconducting structures are oriented with spatial distribution. This variant requires, in particular, no additional structures to produce a magnetic field.
In a particularly preferred method variant, the transverse additional magnetic field is generated by first-time or repeated introduction of the already superconducting coil into the magnet producing the static magnetic field B0. This is particularly simple and inexpensive.
In a further method variant, the additional transverse magnetic field is changed with only one sign to eliminate undesired demagnetization of the superconducting structures.
In a preferred method variant, the additional transverse magnetic field is one single pulse whose amplitude is dimensioned such that the final magnetization of the superconducting components is non-zero. Introduction of one single pulse is a magnetization method which requires little effort from a device standpoint.
In another method variant, the transverse additional magnetic field is a double or multiple pulse, wherein the last pulse has an amplitude and sign such that the final magnetization of the superconducting components is different from zero. This variant can be used to adjust a certain transverse magnetization other than zero and at the same time minimize disturbances resulting from application of the pulses.
In a further method variant, the transverse additional magnetic field is a double pulse, wherein the second pulse has a sign opposite that of the first pulse and the amplitudes of both pulses are approximately equal. The magnetization produced by the first pulse is thereby inverted by the second pulse. A double pulse has the favorable property that eddy currents induced in the neighboring structures are minimized.
In a preferred embodiment of the inventive NMR resonator, the superconducting conductor structures are substantially parallel to the B0 direction to ensure maximum elimination of the Bz component in the sample.
One embodiment of the inventive NMR resonator is characterized in that the material of the superconducting conductor structures is selected such that they remain maximally magnetized after application of the additional magnetic field thereby maximizing the magnetic field in the sample region caused thereby. In this fashion, the magnetization state remains uniquely defined after application of the additional magnetic field.
In a particularly preferred embodiment of the inventive device, the maximum magnetization transverse to the direction of B0 of the superconducting conductor structures is substantially homogeneous over z. This provides particular easy setting of a transverse magnetization which is constant along the z axis and different from zero.
Further advantages of the invention can be extracted from the description and the drawing. The features mentioned above and below can be used in accordance with the invention either individually or collectively in any arbitrary combination. The embodiments shown and described are not to be understood as exhaustive enumeration, rather have exemplary character for describing the invention.
The invention is shown in the drawing and is explained in more detail by means of embodiments.